How to choose memory for a PC

Memory can be a major bottleneck

A shortfall in memory results in a significant drop in both PC capability and
performance. If your computer is slow the first place to check is not the
CPU, but the amount of memory.

How much memory should you install in your PC

Windows 32-bit operating systems can only handle approximately 3GB of
memory no matter how much physical memory you put in, while the
Windows and Linux 64 bit versions do not have this limitation. 64 bit versions of
Windows and Linux are now mainstream so getting at least 4 GB of memory is
worthwhile. 4 GB is sufficient for mainstream tasks.

Quad, Dual and single channels

If you're installing 4 GB, get one memory stick (module) of 4 GB now so you have an easy upgrade in the future by adding another 4 GB stick later. One memory stick will result in your memory running in single channel mode, but the difference between dual and single channel mode seems marginal on synthetic tests and probably unnoticeable in real world applications. Dual channel requires at least 2 sticks. Workstations and servers can have quad channel where you'll need at least 4 sticks to get that benefit.

Memory clock speeds and latencies

Once we've decided on the amount of memory we decide on memory
performance measures. The two variables here are clock speeds and
latencies. You want higher clock speeds and lower latencies or wait times.
Latencies are expressed as a class, and the lower the class number
the better. However If you've got to pay significantly more for
better than standard clock speeds and latencies which is usually the case it's better not to pay up. That saving is
better spent on a faster CPU or graphics solution where a larger
performance gain will be seen. So we recommend standard memory speeds and latencies. Currently that's DDR4 at 2133 MHz Class 15.

Often your CPU chip specifies what speed memory it can handle, get that memory speed.